This achievement is for completing the 4th mission in Chapter 3, called The Keep.
Some of the sections in this level are quite challenging, such as the summoners in the windows of the castle at the start, but the main issue I had was with the end of level bottleneck.
When you get back to the cable car, you are effectively assaulted by a reasonable number of zombies, plus the suicide bombers and 2 elites. The combination of these meant that (on Marksman difficulty, single player), I was always overwhelmed in the corner, even when using the mounted machine gun for as long as possible.
The way I did it, was to trigger the finale (the doors shut on the house), then turn around and run up the hill to the right, all the way to the bridge that is blown (you crossed it earlier). This is where the suicide bombers spawn. What I did was kill the first 4 the instant they spawned (plus one you meet en route).
After this, I then sprinted back down to the machine gun nest, near the house. The tricky bit here is surviving running past the elites - what I did was run back and hide in the keep, and ran out when the first elite was entering so he didn't get chance to shoot at me. By the time you get back to the machine gun, the zombies/elites will be well up the hill, and as such, this gives you loads more time to waste them on the machine gun as they amble towards you (also the suicide guys stopped spawning). It was still tough, but I did it first time this way, unlike 15 failed tries by just camping there. Good luck!
P.S The achievement title is a reference to the call sign used by Richard Burton in the film "Where Eagles Dare". Its a top war film (my all time favourite), and I recommend it if you haven't seen it